The organic solvents used in organic solvent type-coating compositions are evaported and discharged into the atmosphere during the coating operations such as baking, which deteriorates the environments on the earth. Recently, the interlayer coatings and overlayer coatings are constituted of the combination of polyesterpolyols with melamine resin. In this coating line, a large amount of organic solvents is discharged into the atmosphere. When the exhausted organic solvents are burned, a large amount of carbon dioxide are generated and promotes the warming of the earth. Therefore, various attempts have been conventionally made to reduce the amount of the organic solvents to be discharged. For example, there have been proposed high solid content coating compositions wherein a low molecular weight melamine resin is combined with polyesterpolyols and high solid content coating compositions of acrylic type (e.g., Japanese Laid-open Publication (J.P. Kokai) Nos. 61-9461 and 55-123660). However, the minimum content of the organic solvent as attained by such coating compositions is about 40%. Such amount is insufficient in view of the protection of the environment. Further, when polyesterpolyol to be combined with a low molecular weight melamine resin is conventionally prepared, it is very difficult to make lower the molecular weight of the polyesterpolyol and it is very difficult to make lower the viscosity of the coating composition using such polyesterpolyol. In such coating compositions, there are problems in that it is very difficult to provide strong coating which can improve chipping resistance and that the sagging on the vertical surface is remarkable when the coating is made, because the viscosity of the coating composition becomes lower.
There have been widely investigated aqueous coating compositions without using or without substantially using organic solvents or powder coating compositions for the purpose of decreasing the amount of the organic solvents. However, the aqueous coating compositions are poor in water resistance and still need at least about 20% of organic solvents. Since the powder coating compositions need resins having a very higher glass transition temperature, there is a problem in that the coating obtained from the powder coating compositions is brittle and cannot meet the requirements of scuff resistance and chipping resistance as required for overlayer coatings and interlayer coatings.
Therefore, there is a strong demand for coating compositions which meet the requirements of the scuff resistance and chipping resistance of the resultant coating and which can reduce the amount of the organic solvents to a large degree.
On the other hand, there have been conventionally known high solid content coating compositions comprising resins containing a hydroxy group as an essential functional group and curing agents such as melamine resins comprising completely alkoxylated melamine, as high solid content coating compositions suitable for base coating or clear coating compositions. Such high solid content coating compositions contain strong acid catalysts as curing catalysts, in order to improve the curing reaction between the resins and curing agents.
However, when acidic rain falls on the coating formed using such high solid content coating compositions as a clear coating composition, the acid (such as sulfuric acid or nitric acid) in the rain attacks the coating, decomposes it, and then pours the decomposed substances away. As a result, there is a problem in that fine roughness remains on the surface, which can be seen as kind of stain. In order to resolve this problem, there has been proposed clear coating compositions which can be cured by the reaction between a carboxyl group and an epoxy group. In this case, a basic catalyst is used as a curing agent.
However, the base coating compositions use strong acid catalysts. When clear coating compositions containing such basic catalyts are coated on such base coating compositions, there are problems, for example, in that the functions of the basic catalysts and the strong acid catalysts offset each other and that the strong acid catalysts in the base coating compositons react with the epoxy group in the clear coating compositions. Consequently, the curability of the base coating compositions is remarkably reduced, the adhesion of the base coating to the interlayer coating is reduced, and therefore, a new problem appears in that the resultant coating becomes poor in water resistance and chipping resistance.
Therefore, there is also a strong demand for a new coating process for low solvent content type coating compositions, which resolves such drawbacks and reduces the amount of the organic solvents to a large extent.